Brake control system with skid control

ABSTRACT

Wheel velocity is monitored and at the onset of brake application, the system commits wheel velocity signal to memory as a vehicle velocity signal. During brake application, the wheel velocity signal is scaled to appear a fractional part greater than it actually is. As braking lowers actual wheel velocity to a fixed fraction of that remembered vehicle velocity, the upward biased wheel signal will equal the remembered signal. This equality means that the wheel is at a fixed fraction of vehicle speed, hence at a fixed wheel slip percentage. Thereafter, error in the signal balance or equality is used to adjust a modulating device which raises or lowers braking torque to remove or minimize that error. Such modulation of the braking torque adjusts the wheel speed to maintain a fixed proportionality to vehicle speed. By integrating the motion of an inertial mass in the system, vehicle velocity is constantly updated throughout the vehicle deceleration.

United States Patent 1 Koppl et al.

[54] BRAKE CONTROL SYSTEM WITH SKID CONTROL [75] Inventors: Ernest R.Koppl, South Euclid; Harold R. Scibbe, Chardon, all of Ohio [73]Assignee: TRW Inc., Cleveland, Ohio [22] Filed: July 27, 1970 [21] Appl.No.: 58,535

[52] U.S. Cl ..303/2l B, 188/181 C, 303/21 BE, 303/21 F [51] Int. Cl...B60t 8/08 [58] Field of Search....l88/l8l C; 303/20, 21; 317/5;318/52, 326, 328; 324/161; 340/263, 268

[56] References Cited Mueller ..303/2l BB 1 Jan. 30, 1973 Attorney-Hill,Sherman, Meroni, Gross & Simpson [57] ABSTRACT Wheel velocity ismonitored and at the onset of brake application, thesystem commitswheelvelocity signal to memory as a vehicle velocity signal. During brakeapplication, the wheel velocity signal is scaled to appear a fractionalpart greater than it actually is. As braking lowers actual wheelvelocity to a fixed fraction of that remembered vehicle velocity, theupward biased wheel signal will equal the remembered signal. Thisequality means that the wheel is at a fixed fraction of vehicle speed,hence at a fixed wheel slip percentage. Thereafter, error in the signalbalance 0r equality is used to adjust a modulatingdevice which raises orlowers braking torque to remove or minimize that error. Such modulationof the braking torque adjusts the wheel speed to maintain a fixedproportionality to vehicle speed. By integrating the motion of aninertial mass in the system, vehicle velocity is constantly updatedthroughout the vehicle decelerationf 7 Claims, 3 Drawing FiguresPATENTEUJAHOIHB 3.713.704 SHEET 1 [IF 3 INVENTORS f e M557 1?. KOP LPATENTEDJAN 30 I975 SHEET 2 BF 3 mks Z W E PATENTED JAN 3 0 I975 SHEET 3BF 3 BRAKE CONTROL SYSTEM WITH SKID CONTROL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates generally tobraking systems for wheeled vehicles and more particularly to such abraking system with skid control.

2. The Prior Art In prior art arrangements heretofore provided variousmeans have been included in braking systems in SUMMARY OF THE INVENTIONIn accordance with the principles of the present invention, a vehiclebraking system is provided based on the concept that vehicle retardingforce, generated at the wheel-road interface, is maximized when a finitevelocity difference is maintained between the wheel contact area and theroadway. Furthermore, optimum frictional co-efficient for braking isobtained within a nominal wheel slip range. Such wheel slip is normallyexpressed as the ratio of the velocity difference to vehicle velocity.In'this braking concept, the system monitors wheel velocity directly,computes vehicle velocity, and modulates brake pressure to maintain aspecific value of wheel slip. Execution of this concept is appropriatefor the variety of roadway frictional co-efficients normallyencountered. The several components of the system are in operationalmotion during every brake application. Braking'control is retained bythe vehicle operator unless excessive brake pressure causes wheel slipto depart from the range associated with optimum frictional coefficient.

in its broadest scope, it is contemplated that the system will commit awheel velocity signal to memory as a vehicle velocity signal. While thebrake is applied, the wheel velocity signal is scaled to appear afractional part greater than it actually is. Then as braking lowersactual wheel velocity to a fixed fraction of that remembered vehiclevelocity, the upward biased wheel signal will equal the rememberedsignal. This equality means that the wheel is now at a fixed fraction ofvehicle speed, hence at a fixed wheel slip percentage.

Thereafter, error in the signal balance or equality is used to adjust amodulating device which raises or lowers braking torque to remove orminimize that error. Such modulation of the braking torque adjusts thewheel speed to maintain a fixed proportionality to the vehicle speed.

The basic concept can be interpreted electronically by sensing andtranslating wheel rotation to produce a voltage which is proportional towheel speed. When the brakes are applied, a relay is energized todisconnect an operational amplifier from a wheel speed signal and toreconfigure the operational amplifier, thereby producing a voltage whichis the negative of the voltage representing vehicle velocity prior tobraking as obtained from the wheel sensor.

During braking, the vehicle deceleration is sensed by a chassis mountedlinear accelerometer and presented as a voltage. This signal isintegrated by the operational amplifier to produce a signal whichrepresents vehicle velocity, having started at the value derived fromthe wheel sensor when the wheel was rolling freely prior to braking. Thesignal is then continuously reduced through braking by the time-integralof deceleration.

The wheel speed signal and the vehicle velocity signal are scaled andadded to produce an input to an error amplifier. Since the one signal isof an opposite polarity to the other, the addition actually correspondsto the subtraction of the scaled values of vehicle and wheel speed. Theoutput of the error amplifier, which represents the deviation of actualwheel slip from design slip is used to control a brake control actuatorwhich adjusts braking effort to make actual slip equal to the designwheel slip.

In another interpretation of the same basic concept wherein the entiremethod and means constitutes a hydraulic arrangement, instantaneouswheel speed is controlled by wheel cylinder pressure to be a constantfraction of vehicle speed. This is accomplished by porting the flow froma wheel-driven pump through two orifices in series, the first orificebeing larger than the second. Prior to braking, pressure drop across thefirst orifice is converted to spring load by a piston. During braking,the same spring load is compared to pressure drop across a second,smaller orifice, by a control valve. The control valve action willincrease brake pressure, slowing the wheels and the pump, until thepressure drop caused by the reduced flow through the smaller orificeequates to the spring load. Thus, a continuous closed-loop controlaction will govern the speed of the braked wheels as they retard thevehicle.

Pump flow is ported through either or both of the two parallel valvingelements, a throttle valve actuated by master cylinder pressure, and acontrol valve responsive to instantaneous wheel speed. In both cases,valve outlet flows are returned to the pump. The wheelspeed responsivevalve is normally closed until brake action has slowed the wheels to apreset fraction of free rolling speed, i.e., until the desired wheelslip has been attained. Thereafter, controlled openings and closings ofsuch valve are responsible for brake pressure modulation.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat schematic view showingthe principles of the present invention as applied to a wheeled vehicle;

FIG. 2 is a plumbing diagram showing the hydraulic circuitry of thecontrol system utilized in the braking system of FIG. 1;

FIG. 3 is a schematic circuit diagram showing an electronic analogue ofthe brake control system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the principles of thepresent invention are shown in connection with an application to therear wheels of a wheeled vehicle, it will be understood that theprinciples are of general applicability and could be applied to all fourwheels of a typical passenger automobile.

Referring to FIG. 1 there is shown the vehicular wheel drive of atypical motor vehicle such as a passenger car or truck. The apparatusincludes a transmission driving a propeller shaft 11 through adifferential 12 connected by way of the axles 13 and 14 to rear wheelsshown at 16 and 17, respectively. The wheels 16 and 17 have a wheel-roadinterface with a road surface.

Although various types of brakes may be provided including disk brakesor other forms of deceleration means, the present invention is shownillustrated in connection with brakes utilizing shoes 18 actuated by abrake actuator shown at 19 receiving fluid at hydraulic pressure througha conduit labeled P connected to a control 20. An actuator pedal 21 isoperated by the driver of the vehicle and through an appropriate linkage22 operates a master cylinder 23, thereby generating a pressure which isconducted through a conduit labeled P which also leads to the control20. There is also shown a pump 30 which is rotatably driven by a driver31 connected to the propeller shaft 11 by means of a drive belt 32,thereby to rotatably drive the pump 30 as a function of the rotationalspeed of the wheels 16 and 17.

Referring now to FIG. 2, the control assembly is illustrated in the formof a simplified plumbing diagram. The pump 30 previously referred to inconnection with FIG. 1 is a hydraulic pressure pump driven at a speedproportional to wheel speed so that flow delivery is a direct functionof such speed irrespective of the pressure level maintained.

The control assembly includes two speed sensitive valves and an inertialmass, and means to circumvent such control when it is incapable ofmaintaining adequate pressure, to retain vehicle braking.

A pressure output to the wheel brake cylinders is that shown previouslyin FIG. 1 as P and the input pressure from the driver actuated masterbrake cylinder is shown at P Referring to the plumbing diagram of FIG.2, whenever the vehicle is in motion, flow from the wheeldriven pump 30is directed through a first orifice 40 at a piston element 41, therebycreating a pressure differential which is felt by a piston area 42.

In a manner analogous to a speedometer, the piston element 41 moves tothe right, using the directional orientation of FIG. 2, in the samedirection as the forward vehicle motion indicated by an arrow 43,thereby compressing a spring 44 as the pump and wheel speed increase.Thus, the compressed spring load is the stored equivalent of freerolling wheel speed, and hence of vehicle speed.

The controller 20 is provided with a plurality of passages 47, 48, 49,50 and 51, which are utilized so that pump flow through a second,smaller orifice 52, establishes a pressure differential across anannular area 53-54 on a valving element 56.

Prior to braking, the pressure differential on the annular area 53-54 issufficiently large to urge the valve 56 against its limiting stop 57,thereby closing a flow passage'58 and to sustain the compression load ofthe spring 44 on an extension 59 of the valve 56. Prior to braking, theareas 42 and in the vicinity of the orifice 40, are subjected to pumppressure P,,.

By virtue of a non-return valve 60, areas of the controller designatedat 61, 62, 63 and 64 and also 66 and 67 are subjected to an intermediatepressure P,.

Areas designated at 47, 48, 68, 69 and 70, as well as a port 71 to thewheel cylinders are subjected to the pressure P,,,. At this time, thepressure P,,, is quite small, sufficient only to hold a throttle valve72 away from a seat 73 to permi return of pump flow to sump 74.

As the throttle valve 72 throttles return flow through a port 76 inresponse to driver input of master cylinder pressure P on an area 77,the pressures P,,, P, and P, rise proportionately.

The increasing P acting in the wheel cylinders to produce brake torque,initiates wheel deceleration and a reduction of pump speed and flow.

It is necessary to decouple the initial speed setting mechanism, afterbraking effort has begun, because the equivalent speed springcompression was derived from pump flow at free rolling wheel speed andwheel speed ceases to measure vehicle speed as wheel slip begins.Accordingly, a larger diameter extension 78 on the piston element 41 andin conjunction with the check valve 60, hydraulically locks the springcompression as the wheel deceleration begins. The reduction in pump flowthrough the first orifice 40 no longer directly coning the predeterminedwheel slip, the pressure dif- Y ferential from the second orifice 52acting on the annular area 53-54 of the valve 56 will just sustain thespring compression. Thus, the locked spring load automatically becomesthe correct command for braked wheel speed with the desired fractionalwheel slip. Should continued wheel deceleration result in excessivewheel slip, the further reduction of pump flow will reduce the pressuredifferential sustaining the spring compression, and the valve 56 willmove to open the port 58 and prevent further rise of the pressure P,,,and further reduction of the instantaneous wheel speed. Thus, the valve56 continues to modulate brake pressure P,,, to maintain a pump flow andpressure differential to sustain the spring compression. The fractionalreduction in instantaneous wheel speed, at the time of the port 58opening, is equivalent to the ratio of flow areas established in thefirst and second orifices 40 and 52. Orifice 45 serves to limitoscillatory motion of valve 56, thereby providing the damping necessaryfor system stability.

Since the effect of wheel braking reduces vehicle speed, it is apparentthat if the spring compression speed command were to remain unchanged,vehicle speed would decrease only to that of the controlled wheel speed.This is prevented by utilizing an inertial mass 79 to control subsequentleftward motion of the piston element 41 so that the speed command isreduced at a rate proportional to vehicle deceleration. Motion of theinertial mass 79 against a spring 80 uncovers a flow area of a shapedport 81 to relieve the hydraulic lock of the area 66 into the area 62.Now able to respond to the pressure differential on areas 42 and 78, thepiston element 41 moves to the left as vehicle speed is reduced.

In this combination, the larger piston 78 integrates flow through theshaped port to find the equivalent speed decrement for the currentvehicle deceleration rate. In this way, spring load is continuouslymodified to reflect actual vehicle speed while being used to controlwheel speed.

Since the pump is incapable of generated pressure as the wheels come toa stop, brake control must eventually revert to the master cylinder. Aport 82 is provided for this purpose and is located for communicationwith the second orifice 52 as the piston 41 is positioned equivalent toa low wheel speed. When such low pump speed can no longer generate therequired braking pressure P,,,, the area shown at 83 is vented throughthe port 82 into the passage 47, and the piston 84 will be mechanicallypushed by the extension 86 of the throttle valve 72 against a bufferpiston 87 to maintain brake pressure in the area 70, and to the wheelcylinders 19, 19.

A practical difficulty would exist if the two orifices 40 and 52 werefixed in size. For the system to function over a large vehicle speedrange, but with a minimal pressure differential range, these orificesare designed as unequal width slots in the bore retaining piston 41 andare uncovered in length by the piston displacement. As the vehicle speedincreases, both orifices 40 and 52 increase in size while maintaining afixed ratio of flow areas.

In basic concept, the structure thus provided monitors wheel velocityand at the onset of brake application, the system commits the wheelvelocity signal to memory as a vehicle velocity signal. While the brakeis applied, the wheel velocity signal is scaled to appear a fractionalpart greater than it actually is. Then as braking lowers actual wheelvelocity to a fixed fraction of that remembered vehicle velocity, theupward biased wheel signal will equal the remembered signal. Thisequality means that the wheel is now at a fixed fraction of vehiclespeed, hence at a fixed wheel slip percentage. Thereafter, error in thesignal balance or equality is used to adjust a modulating device whichraises or lowers braking torque to remove or minimize that error. Suchmodulation of the braking torque adjusts the wheel speed to maintain afixed proportionality to vehicle speed. By integrating the motion of theinterial mass in the system, vehicle velocity is constantly updatedthroughout the vehicle deceleration.

By way of summary, it will be noted that the throttle valve 72 serves asa pump flow by-pass around the normally closed control valve prior tomaster cylinder 23 pressure rise. It thus prevents brake pressure riseduring nonbraking wheel rotation. Under normal low slip brakingconditions, the throttle valve 72 throttles pump return flow in responseto master cylinder pressure so that pump and wheel cylinder pressuresare substantially equal to master cylinder pressure.

When such pressures result in excessive wheel slip, the wheel-speedresponsive valve action begins. Should master cylinder pressure continueto rise, due to driver input, the throttle valve by-pass port 76 closescompletely and transfers brake control to the wheelspeed sensitivevalve.

In order to maintain a constant ratio of wheel-tovehicle speed asvehicle speed decreases, the spring load is reduced in accordance withvehicle deceleration. This is accomplished by venting the hydraulicallylocked piston 41, the same piston 41 that first retained the spring loadof the spring 44, through the orifice 81 whose flow area is the functionof the inertial mass 79 reacting against the spring 80. Thus,instantaneous vehicle speed, throughout the braking period is comptitedby the integration of linear vehicle deceleration with respect to time.Inherent non-linearities of the system are accommodated by the variableshape of the orifice 81.

Control circuit pressure is transferred to the wheel cylinder circuit bythe buffer piston 87 which isolates one fluid circuit from the other. Toprevent loss of braking capability due to failure in the controlcircuit, extended motion of the driver actuated throttle valve 72 canmechanically push the buffer piston 87 to generate pressure at the wheelcylinders 19, 19.

It should be noted that the pressure areas on the throttle valve 72 andon the buffer piston 87 may be pre-selected to afford a desiredmechanical advantage, thereby to boost wheel cylinder pressurehydraulically with respect to master cylinder pressure. It is alsopossible that such relative areas may be selected to reduce wheelcylinder pressure, for example, if a system were desired whereinpressure to one set of wheels might be supplied at a different pressurethan to another set of wheels.

That basic system can also be interpreted by an electronic analogue ofthe hydraulic version illustrated in FIGS. 1 and 2.

' Referring now to FIG. 3, it will be noted that wheel rotation issensed and translated by the sensor and conversion circuits to produce avoltage at A which is proportional to wheel speed. While the brakes arenot applied, the deenergized relay (dashed lines) connects R and R, assignal and feedback resistors to the operational amplifier so as to forma voltage invertor, and charges the capacitor C to the negative of thevoltage at A.

When the brakes are applied, the relay is energized to disconnect theoperational amplifier 100 from the wheel speed signal and re-configurethe operational amplifier 100 with the capacitor C, and the resistor Ras an integrator. The voltage previously stored on the capacitor Cbecomes the initial output of the integrator. This voltage is thenegative of the voltage representing vehicle velocity prior to brakingas obtained from the wheel sensor.

During braking the vehicle decelerationtnot the wheel deceleration) issensed by a chassis mounted linear accelerometer identified by legendand shown schematically and is presented as a voltage at B. This signalis integrated by the operational amplifier 100 to produce a signal at Cwhich represents vehicle velocity, having started at the value derivedfrom the wheel sensor when the wheel was rolling freely prior tobraking. The signal at C is then continuously reduced through braking bythe time-integral of deceleration.

The signals at A and C are scaled and added by R and R to produce theinput to the error amplifier. It will be remembered that C is ofopposite polarity to A and hence this addition actually corresponds tothe subtraction of the scaled values of vehicle and wheel speed. Thevalues of R and R, are chosen so as to produce zero error signal whenthe measured wheel speed is less than the calculated vehicle speed by aspecific fraction known as'the design slip.

The output of the error amplifier, shown at 101, which represents thedeviation of actual wheel slip from design slip, is used to control abrake control actuator 102 which adjusts braking effort to make actualslip equal to the design wheel slip.

There is thus provided in the two versions of the braking concept hereindisclosed, a system which monitors wheel velocity directly, computesvehicle velocity, and modulates brake pressure to maintain a specificvalue of wheel slip.

We claim as our invention:

1. The method of braking a vehicle so that by integrating the motion ofan inertial mass in the system vehicle velocity is constantly updatedthroughout vehicle deceleration and which method includes the steps ofa. monitoring wheel velocity whenever the wheels of the vehicle arerotating by driving a supply of fluid through a circuit in the form of astream as a function of vehicle speed,

. at the onset of a braking application committing a signal to memorywhich is a function of the wheel velocity thereby constituting a vehiclevelocity signal by converting a pressure drop at one point in saidcircuit into a spring load,

c. while the brake is applied during the braking application scaling acontinuing wheel velocity signal upwardly to appear a fractional partgreater than it actually is by comparing the spring load to a pressuredrop at a second point in said circuit by a valving action,

cl. then allowing the scaled-up wheel velocity signal to decrease asbraking during the braking application lowers actual wheel velocity to afixed fraction of the remembered wheel velocity so that the upwardbiased wheel signal will balance the remembered signal by continuouslyreducing the remembered signal to correspond to the reducing vehiclevelocity by the time integral of vehicle deceleration which reduction iseffected by reducing the spring load in accordance with vehicledecleration,

whereupon the wheel rotates at a fixed fraction of vehicle speedcorresponding to a fixed wheel slip percentage,

e. effecting continuously integration of vehicle deceleration whichconstantly updates vehicle velocity,

f. and adjusting a modulating device as a function of the error in thesignal balance by applying braking effort as a function of hydraulicforce,

thereby to raise or lower the braking torque to minimize such error andadjusting wheel speed to maintain a fixed proportionality to vehiclespeed.

2. A vehicular braking system comprising, means forming a hydrauliccircuit,

a wheel-driven pump in said circuit for driving a supply of liquid inthe form of a stream through said circuit as a function of wheel speed,

first and second orifices in said circuit disposed in series for portingthe flow from said pump, said first orifice being larger than saidsecond orifice,

piston means having motive surfaces responsive to the pressure dropacross said first orifice,

spring means engageable with said piston means, whereby the pressuredrop across said first orifice is converted to a spring load,

a control valve in said circuit downstream of said second orifice forcomparing the pressure drop across said second smaller orifice to saidspring load,

brake actuating means in said circuit for slowing the wheels, saidcontrol valve action increasing brake pressure and slowing the wheelsand the pump until the pressure drop caused by flow through said smallerorifice equates to said spring load, whereby a closed-loop controlaction will govern the speed of the braked wheels as they retard thevehicle.

3. A hydraulic vehicle braking control system comprising means formingan hydraulic circuit,

a wheel-driven pump in said circuit for driving a supply of liquid inthe form of a stream,

first and second orifices in series,

a spring and a piston means engageable with said spring and havingmotive surfaces responsive to the pressure drop across said firstorifice so that prior to braking, the pressure drop across said firstorifice is converted into a spring load,

two parallel valving elements in said circuit including a throttle valveactuated by master cylinder pressure and a control valve responsive toinstantaneous wheel speed,

valve outlet flows from both of said valves being returned to said pump,

said control valve being normally closed until brake action has set thewheels to a pre-set fraction of free rolling speed, i.e., wheel slip,but opening thereafter to effect brake pressure modulation,

said throttle valve operating as a pump flow bypass around said normallyclosed control valve prior to master cylinder pressure rise,

and venting means for said piston means for reducing the spring load inaccordance with vehicle deceleration by maintaining a constant ratio ofwheel to vehicle speed as vehicle speed decreases.

4. A hydraulic vehicle braking control system as defined in claim 3 andfurther characterized by said venting means comprising an orifice whoseflow area is the function of an inertial mass reacting against a secondspring in response to vehicle deceleration, whereby instantaneousvehicle speed throughout the braking period is computed by theintegration of linear vehicle deceleration with respect to time.

5. An hydraulic vehicle braking control system as defined in claim 4 anda buffer piston responsive to control circuit pressure and operative totransfer such pressure to the wheel cylinder circuit of the brakingsystem, said buffer piston being axially aligned with said throttlevalve whereby extended motion of the driver-actuated throttle valve canmechanically push the buffer piston to generate pressure at the wheelcylinders.

6. An hydraulic vehicle braking control system as defined in claim 3 andfurther characterized by said throttle valve throttling pump return flowin response to master cylinder pressure under normal low slip braking sothat the pump and wheel cylinder pressures are substantially equal tomaster cylinder pressure, or some ratio thereof,

said throttle valve operating to close the by-pass port completely inthe event of master cylinder pressure increase due to operator inputwhereby brake control will be transferred to the wheelspeed sensitivecontrol valve.

7. The method of braking a vehicle so that by integrating the motion ofan inertial mass in the system, vehicle velocity is constantly updatedthroughout vehicle deceleration and which method includes the steps ofa. monitoring wheel velocity whenever the wheels of the vehicle arerotating by driving a supply of fluid through a circuit in the form of astream as a function of vehicle speed,

b. at the onset of a braking application committing a signal to memorywhich is a function of the wheel velocity thereby constituting a vehiclevelocity signal by converting a pressure drop at one point in saidcircuit into a spring load,

c. while the brake is applied during the braking application scaling thewheel signal upwardly to appear a fractional part greater than itactually is by comparing the spring load to a pressure drop at a secondpoint in said circuit by a valving action,

d. then allowing the scaled up wheel signal to decrease as the brakingapplication lowers actual wheel velocity to a fixed fraction of theremem-- bered wheel velocity so that the upward biased wheel signal willbalance the remembered signal,

so that the wheel is at a fixed fraction of vehicle speed correspondingto a fixed wheel slip percentage,

e. continuously reducing the remembered signal to correspond to thereducing vehicle velocity by the 7 thereby to raise or lower the brakingtorque to minimize such error and adjusting wheel speed to maintain afixed proportionality to vehicle speed.

1. The method of braking a vehicle so that by integrating the motion ofan inertial mass in the system vehicle velocity is constantly updatedthroughout vehicle deceleration and which method includes the steps ofa. monitoring wheel velocity whenever the wheels of the vehicle arerotating by driving a supply of fluid through a circuit in the form of astream as a function of vehicle speed, b. at the onset of a brakingapplication committing a signal to memory which is a function of thewheel velocity thereby constituting a vehicle velocity signal byconverting a pressure drop at one point in said circuit into a springload, c. while the brake is applied during the braking applicationscaling a continuing wheel velocity signal upwardly to appear afractional part greater than it actually is by comparing the spring loadto a pressure drop at a second point in said circuit by a valvingaction, d. then allowing the scaled-up wheel velocity signal to decreaseas braking during the braking application lowers actual wheel velocityto a fixed fraction of the remembered wheel velocity so that the upwardbiased wheel signal will balance the remembered signal by continuouslyreducing the remembered signal to correspond to the reducing vehiclevelocity by the time integral of vehicle deceleration which reduction iseffected by reducing the spring load in accordance with vehicledecleration, whereupon the wheel rotates at a fixed fraction of vehiclespeed corresponding to a fixed wheel slip percentage, e. effectingcontinuously integration of vehicle deceleration which constantlyupdates vehicle velocity, f. and adjusting a modulating device as afunction of the error in the signal balance by applying braking effortas a function of hydraulic force, thereby to raise or lower the brakingtorque to minimize such error and adjusting wheel speed to maintain afixed proportionality to veHicle speed.
 1. The method of braking avehicle so that by integrating the motion of an inertial mass in thesystem vehicle velocity is constantly updated throughout vehicledeceleration and which method includes the steps of a. monitoring wheelvelocity whenever the wheels of the vehicle are rotating by driving asupply of fluid through a circuit in the form of a stream as a functionof vehicle speed, b. at the onset of a braking application committing asignal to memory which is a function of the wheel velocity therebyconstituting a vehicle velocity signal by converting a pressure drop atone point in said circuit into a spring load, c. while the brake isapplied during the braking application scaling a continuing wheelvelocity signal upwardly to appear a fractional part greater than itactually is by comparing the spring load to a pressure drop at a secondpoint in said circuit by a valving action, d. then allowing thescaled-up wheel velocity signal to decrease as braking during thebraking application lowers actual wheel velocity to a fixed fraction ofthe remembered wheel velocity so that the upward biased wheel signalwill balance the remembered signal by continuously reducing theremembered signal to correspond to the reducing vehicle velocity by thetime integral of vehicle deceleration which reduction is effected byreducing the spring load in accordance with vehicle decleration,whereupon the wheel rotates at a fixed fraction of vehicle speedcorresponding to a fixed wheel slip percentage, e. effectingcontinuously integration of vehicle deceleration which constantlyupdates vehicle velocity, f. and adjusting a modulating device as afunction of the error in the signal balance by applying braking effortas a function of hydraulic force, thereby to raise or lower the brakingtorque to minimize such error and adjusting wheel speed to maintain afixed proportionality to veHicle speed.
 2. A vehicular braking systemcomprising, means forming a hydraulic circuit, a wheel-driven pump insaid circuit for driving a supply of liquid in the form of a streamthrough said circuit as a function of wheel speed, first and secondorifices in said circuit disposed in series for porting the flow fromsaid pump, said first orifice being larger than said second orifice,piston means having motive surfaces responsive to the pressure dropacross said first orifice, spring means engageable with said pistonmeans, whereby the pressure drop across said first orifice is convertedto a spring load, a control valve in said circuit downstream of saidsecond orifice for comparing the pressure drop across said secondsmaller orifice to said spring load, brake actuating means in saidcircuit for slowing the wheels, said control valve action increasingbrake pressure and slowing the wheels and the pump until the pressuredrop caused by flow through said smaller orifice equates to said springload, whereby a closed-loop control action will govern the speed of thebraked wheels as they retard the vehicle.
 3. A hydraulic vehicle brakingcontrol system comprising means forming an hydraulic circuit, awheel-driven pump in said circuit for driving a supply of liquid in theform of a stream, first and second orifices in series, a spring and apiston means engageable with said spring and having motive surfacesresponsive to the pressure drop across said first orifice so that priorto braking, the pressure drop across said first orifice is convertedinto a spring load, two parallel valving elements in said circuitincluding a throttle valve actuated by master cylinder pressure and acontrol valve responsive to instantaneous wheel speed, valve outletflows from both of said valves being returned to said pump, said controlvalve being normally closed until brake action has set the wheels to apre-set fraction of free rolling speed, i.e., wheel slip, but openingthereafter to effect brake pressure modulation, said throttle valveoperating as a pump flow by-pass around said normally closed controlvalve prior to master cylinder pressure rise, and venting means for saidpiston means for reducing the spring load in accordance with vehicledeceleration by maintaining a constant ratio of wheel to vehicle speedas vehicle speed decreases.
 4. A hydraulic vehicle braking controlsystem as defined in claim 3 and further characterized by said ventingmeans comprising an orifice whose flow area is the function of aninertial mass reacting against a second spring in response to vehicledeceleration, whereby instantaneous vehicle speed throughout the brakingperiod is computed by the integration of linear vehicle decelerationwith respect to time.
 5. An hydraulic vehicle braking control system asdefined in claim 4 and a buffer piston responsive to control circuitpressure and operative to transfer such pressure to the wheel cylindercircuit of the braking system, said buffer piston being axially alignedwith said throttle valve whereby extended motion of the driver-actuatedthrottle valve can mechanically push the buffer piston to generatepressure at the wheel cylinders.
 6. An hydraulic vehicle braking controlsystem as defined in claim 3 and further characterized by said throttlevalve throttling pump return flow in response to master cylinderpressure under normal low slip braking so that the pump and wheelcylinder pressures are substantially equal to master cylinder pressure,or some ratio thereof, said throttle valve operating to close theby-pass port completely in the event of master cylinder pressureincrease due to operator input whereby brake control will be transferredto the wheelspeed sensitive control valve.